What is an adjective for nightmare?

There isn't one single perfect adjective for "nightmare" because it depends on what aspect you want to emphasize. Here are some options based on different connotations:

Scary/Frightening:

* Nightmarish: This is the most direct and common adjective.

* Terrifying

* Horrifying

* Chilling

* Dreadful

Disturbing/Troubling:

* Unsettling

* Eerie

* Distressing

* Haunting

Unpleasant/Uncomfortable:

* Oppressive

* Grotesque

* Surreal

* Bizarre

Other options:

* Nightmarishly realistic: Emphasizes the realistic nature of the nightmare.

* Recurring nightmare: Emphasizes that the nightmare is frequent.

* Living nightmare: Emphasizes the feeling of being trapped in a nightmare.

The best adjective will depend on the specific context you are using it in.